Several systems have been suggested for such a measuring of the flow, such as by means of yielding guard plates being pushed more or less backwards in response to the force they are subjected to by an incoming falling flow of the granular material. It has, however, been recognized that such mechanical measuring methods are too uncertain and that it is possible to employ a more advanced measuring technique based on radioactive radiation. Thus it has been found that it is possible to achieve a well-defined expression of the mass flow of a granular material by said flow passing a measuring area, where a radioactive radiation is emitted from one side of said measuring area towards an opposing side where a receiver detects the radiation and continuously detects the amount of radiation absorbed by the grain flow. In this manner it is possible to determine the mass flow.
Although it is thereby possible to employ radioactive sources which are in fact of a neglectable size, the Authorities have declared that such sources should be avoided because they require so much inspection that an inspection of thousands of moving units would be completely unrealistic.
According to the invention it has been recognized that it is possible to use an officially acceptable type of radiation-based mass determination. In other words it is possible to use electromagnetic microwaves in a frequency area where a predetermined quantity of radiation has been allowed. Besides it is by the present invention sufficient to employ a power of the magnitude of 1 W, whereby suitable shieldings may secure that the radiation problems are reduced to an acceptable level.
It is known from U.S. Pat. No. 4,628,830 to perform a continuous determination of the mass flow in a flow of granular material by means of microwaves. However, this publication deals only with absorption of the wave energy caused by the water content in a coal powder fed to a burner in a power station. The measuring is performed on a falling flow of the material in a pipe where a microwave generator is placed opposite a receiver. In this manner it is possible to measure the amount of energy absorbed in the material, or rather to obtain an expression of the variations applying to the mass flow and thereby to calibrate said variations into an expression of the mass flow. A measuring of the water flow is aimed at, where said water flow in a predetermined material represents the flow of the material itself for a predetermined water content.
The above is possible as long as the material in question is almost homogeneous. A fundamental condition applies, however, to combine harvesters, namely, that the measuring device must be able to operate with various types of material which in no way is homogeneous. The major advantage obtained by the use of the above radioactive radiation is indeed that it is possible to operate with a well-defined calibration of the equipment for various types of grains and seeds.
A substantially analogous use of microwaves does not provide a similar result. Tests performed on microwaves of the type being commercially used exactly for emission of energy into wet substances turned out to be extremely unfortunate, for instance in connection with absorption of energy for heating products in microwave ovens, as it turned out to be impossible by means of one and the same equipment to obtain merely tolerably correct measurements of various mass flows of various granular materials.
It has, however, nevertheless become possible by the invention to base the measurings on the use of microwaves. A method of determining the mass flow speed of a granular material, such as grains, through a channel by means of electromagnetic waves, microwaves from a transmitter to a receiver is characterised in that the attenuation and/or the phase-shift for the main signal and optionally the reflection are measured, the amplitude and the phase shift being measured by comparing the main signal through the material with a reference signal of the same frequency, the reference signal being provided by comparing the output of the transmitter with an injection signal, said compared signal being transmitted through a separate connection to the receiver.
The increased frequency, such as 10 GHz compared to 2 to 3 GHz, results in a considerably higher radiation reflection without considerably influencing the radiation absorption in the material. In other words, the attenuation can be predominantly ascribed to the reflection. At the same time the undesired reflection from the walls of the chamber is increased, and under unchanged conditions the latter renders it almost impossible to obtain useful results.
The associated minimizing of the wall reflection can be obtained in several ways optionally in combination. The chamber can be structured so as not exactly to facilitate reflections towards the measuring window, and it can be coated with radiation-absorbing material, such as sheet material of plastics with carbon powder cast therein. A preferred, although rather complicated possibility is to structure the transmitter aerial system in such a way that the radiation is directed sharply towards the measuring window, whereby only a minor amount of primary radiation causes wall reflections. Good results are obtained by means of slot aerials and focusing parabolic reflectors.
Furthermore, it is important to arrange the measuring chamber in a steady environment. Combine harvesters comprise many metal parts moving relative to one another, and as metal is a good conductor for microwaves, such parts can cause disturbances in the measuring field adjacent the measuring chamber. Although the measuring field adjacent the measuring chamber is shielded, the external forces may, however, manifest themselves to such an extent that a high measuring accuracy aimed at is reduced in case significant vibrations apply. It has surprisingly been found that the measuring chamber is most suitably placed on the location where the radioactive measuring system was previously placed.
On this measuring location, namely, at the top of a pipe bending on a grain channel hoop, the radioactive system aimed at an almost homogeneous distribution of the material transverse to the grain flow, and this is another advantageous aspect of the technique using said measuring location according to the invention because this technique also turned out to operate in the best possible manner with a homogeneous distribution of material. The latter would be of no importance or at least far less importance in connection with measurings based on absorption.
A further incentive for increasing the frequency of the microwaves by the invention is that in order to obtain the desired reflection effect from the various types of grains and seeds it is necessary to take into account that some of these products, such as grass seeds, are of such a small grain size that the grain diameter is smaller than the wavelength of ordinary microwaves for heating purposes and for measuring absorption attenuation, respectively. In view thereof it is according to the invention preferred to operate with a frequency of approximately 22 GHz, i.e. approximately 10 times higher than the frequency for ordinary microwaves, and consequently it is additionally obvious that one should concentrate on attenuation measurings based on reflection rather than absorption.
It is, of course, correct that it is impossible to ignore the attenuation caused by an absorption of the microwave energy in the passing material due to the water content thereof. The importance thereof can indeed be weakened by the use of higher frequencies, but the absorption effect is still a significant factor. Accordingly, it has been accepted that for a good measuring accuracy it is advantageous to perform a supplemental determination of the water content in the measuring mass by means of an independent measuring equipment in or close to the measuring site for the mass flow.